1. Technical Field
The present invention is related to an inverter for a motor driver. In particular, the present invention is related to an inverter circuit for a motor driver which drives a motor by controlling a direct current using a plurality of bridge connected switching elements using a digital signal.
2. Description of Related Art
Generally, an inverter for a three-phase motor driver drives a motor by converting a direct current to a three-phase alternating signal using six bridge connected switching elements. Here an IGBT (Insulation Gate type Bipolar Transistor), or MOSFET, etc. are generally used as such a switching element.
A typically known example of a current power supply (100), an inverter circuit (101) comprised by six bridge connected switching elements, and a three-phase motor (104) driven by a three-phase alternating signal (103) generated by the inverter circuit is shown in FIG. 1.
Generally, a PWM (pulse-width modulation) signal is used for controlling a switching element and an alternating signal is output by a switch operation of the switching element according to the PWM signal. In the case of a three-phase motor, the rotation of the motor is controlled by supplying three-phase alternating signals obtained from six groups of switching elements to a motor. Specifically, it is possible to control the number of rotations and torque of a motor by controlling the amplitude voltage and the frequency of a three-phase alternating signal output from an inverter circuit using a PWM signal.
The principle for obtaining a three-phase alternating signal of a voltage amplitude variable using an inverter circuit according to a PWM signal is explained below using FIG. 2. A triangular wave of a reference signal (201) and input voltages (202a˜202c) of since waves each phase-shifted by a 120 degree phase are compared using a voltage comparator. Signals (203a˜203c) U, V and W from the voltage comparator each become a PWM signal according to the input voltages (202a˜202c) of the since waves. That is, the pulse width of a PWM signal changes due to a change in the amplitude of the sine wave input voltages (202a˜202c), and the amplitude voltage of an alternating signal output by the inverter circuit changes due to a change in the ON/OFF ratio of a switching element controlled by the PWM signal.
In addition, if the frequency of the sine wave input voltages the (202a˜202c) is changed, the pulse density of the PWM signal changes and the frequency of an alternating signal output by the inverter circuit changes due to a change in the ON/OFF frequency of a switching element controlled by the PWM signal.
Since the amount of current that can be controlled by one switching decreases because the switching frequency of an ON/OFF of a switching element increases when the frequency of a triangular wave of a reference signal is increased, it becomes possible to control a current ripple of an alternating signal caused by switching and a magnetic noise of a motor.
With regard to the above mentioned background, there are literatures such as Japanese Patent Laid Open H06-225527 and Japanese Patent Laid Open 2003-530062.
However, a problem arises when such that overall efficiency of the switching elements decreases when the current consumed by the switching elements themselves increases due to an increase in the ON/OFF switching frequency of the switching element.
As described above, the switching elements and a control method of the switching elements are important main components in an inverter circuit for a motor driver from the viewpoint of functionality, efficiency, and cost. In an inverter circuit for a motor driver, making both functionality and cost compatible without losing overall efficiency has become an important technological issue.
Generally, there are three means for providing a switching element with small scale and high efficiency: (1) decreasing the current value for switching the switching element ON/OFF, (2) reducing the ON voltage, and (3) reducing the switching time.
(1) If the voltage level of a direct current power source input to an inverter is increased, the voltage level required for obtaining the same rate of power within the motor drops and therefore it is possible to reduce the current value of the current which the switching element uses to perform ON/OFF. However, because the voltage applied to a switching element increases and the amplitude value of an alternating signal output by an inverter circuit increases, it becomes necessary to increase the insulation resistance between the switching element, the inverter circuit and between coils within the motor. Increasing insulation resistance leads to increases in scale of the inverter circuit and motor device. In addition, because a high voltage current potential becomes necessary for inputting to the inverter circuit, if a booster circuit is introduced before the inverter, this may lead to a significant increase in the cost of the device, which is a problem.
(2) An ON voltage of the switching element is basically determined by the physical characteristics of a switching element. For example, in the case where the switching voltage is 200V or less, it becomes possible to use a MOSFET with a comparatively low ON voltage as a switching element instead of an IGBT. Generally, when the switching voltage of a solid electronic device such as an IGBT or MOSFET is increased, the ON voltage increases. Therefore, it is effective to reduce the switching voltage as much as possible in order to reduce the scale of a switching element.
(3) In order to reduce the switching time of an inverter circuit, it is necessary to increase the transition speed of a PWM signal input to the inverter circuit. In order to increase the transition speed of a PWM signal connected to the gate of a switching element, it is necessary to reduce the gate capacitance of the switching element serving as a load or increase the driving capability of the PWM signal. Generally, the gate capacitance increases when the strength to voltage of a switching element increases. When the gate capacitance increases a problem occurs whereby the current consumed by a switching element itself increases leading to an overall decrease in efficiency. Therefore, it is effective to reduce the switching voltage as much as possible in order to maximize the efficiency of a switching element.
As described above, reducing the size of the switching device is effective by reducing the switching voltage. On the other hand, when the switching voltage is decreased, the current required for driving a motor increases and therefore it becomes necessary to increase the capacitance of the switching element (the number of switching elements), and a small scale and high efficiency inverter circuit for driving a motor cannot be obtained.
In FIG. 1, an example is shown using the PWM signal obtained by comparing a reference triangular wave and an input sine wave signal using a voltage comparator as a control signal of the switching element of an inverter circuit for driving a motor. However, instead of a PWM signal it is also possible to control the switching element by the signal obtained using the input sine wave signal as a pulse density modulation (PDM) using a one-bit delta-sigma modulator. In Japanese Patent Laid Open H06-225527, an example is shown of driving a single phase inductive motor using a signal obtained by modulating the input sine wave signal as a control signal of the switching element using a primary one-bit delta-sigma modulator. In Japanese Patent Laid Open 2003-530062, an example is shown of controlling a switching element of a resonance type switching power supply using the PDM signal from a delta-sigma converter.
However, even in the case where a switching element is controlled using a signal obtained by the PDM, although it is effective to reduce the size of the switching element by reducing the switching voltage, the current required for driving a motor increases when the switching voltage is decreased and therefore, it becomes necessary to increase the capacitance of the switching element (number of switching elements), and a small scale and high efficiency inverter circuit for driving a motor is not possible which does serve to solve this conflicting problem.
As has been explained above, reducing the switching voltage is effective in order to reduce cost without compromising efficiency and the functions required in the inverter circuit for driving a motor. However, the problem exists of not being able to reduce the size of a switching element due to an increase in the current required for driving a motor when the switching voltage is decreased, or the problem of requiring an increase in the capacitance of a switching element (the number of the switching elements) in order to obtain the current required for driving a motor when the switching voltage is decreased leading to an increase in the current consumed by the switching element itself and a drop in overall efficiency.